1. The Field of the Invention
The present invention relates generally to x-ray tubes. More particularly, embodiments of the present invention relate to an x-ray tube having the capability to control the position, size and shape of focal spots on an anode target.
2. The Relevant Technology
X-ray producing devices are extremely valuable tools that are used in a wide variety of applications, both industrial and medical. For example, such equipment is commonly used in areas such as diagnostic and therapeutic radiology; semiconductor manufacture and fabrication; and materials analysis and testing.
While used in a number of different applications, the basic structure and operation of x-ray devices is similar. X-rays, or x-radiation, are produced when electrons are produced, accelerated to a high speed, and then stopped abruptly. Typically, this entire process takes place within a vacuum formed within an x-ray generating tube. An x-ray tube ordinarily includes three primary elements: a cathode assembly, which is the source of electrons; an anode, which is axially spaced apart from the cathode and oriented so as to receive electrons emitted by the cathode; and some mechanism for applying a high voltage for driving the electrons from the cathode to the anode. Usually, the cathode assembly is composed of a metallic cathode head having a cathode cup. Disposed within the cathode cup is a filament that, when heated via an electrical current, emits electrons.
The three x-ray tube elements are usually positioned within an evacuated glass tube and connected within an electrical circuit. The electrical circuit is connected so that the voltage (generation element can apply a very high voltage (ranging from about ten thousand to in excess of hundreds of thousands of volts) between the anode and the cathode. This high voltage differential causes the electrons that are emitted from the cathode filament to accelerate at a very high velocity towards an x-ray xe2x80x9ctargetxe2x80x9d positioned on the anode in the form of a thin stream, or beam. The x-ray target has a target surface (referred to as the focal track) that is comprised of a refractory metal. When the electrons strike the target surface, the kinetic energy of the striking electron beam is converted to electromagnetic waves of very high frequency, i.e., x-rays. The resulting x-rays emanate from the anode target surface, and are then collimated through a window formed in the x-ray device for penetration into an object, such as an area of a patient""s body. As is well known, the x-rays that pass through the object can be detected and analyzed so as to be used in any one of a number of applications, such as x-ray medical diagnostic examination or material analysis procedures.
The area upon which the electron beam is concentrated when it strikes the anode target surface, or focal track, is referred to as the xe2x80x9cfocal spot.xe2x80x9d In most x-ray applications, it is important that the local spot have a specific size and/or shape so as to result in the generation of an x-ray signal that provides an acceptable image quality. This xe2x80x9cfocusingxe2x80x9d of the electron beam is provided primarily at the cathode, which constrains the emitted electron cloud and accelerated electron stream in a manner so as to result in a focal spot having a specific size and shape.
In addition to the need for a focused electron beam, in some applicationsxe2x80x94such as diagnostic radiology for examplexe2x80x94there is a need to generate two or more different x-ray beams having different energy characteristics, and/or two or more x-ray beams that have different angles of incidence upon the area being analyzed, such as the patient""s body. In general, this can be achieved by providing two or more separate focal spots on the focal track. Each focal spot (i.e., point of impact of electrons) will thus generate a separate and distinct x-ray signal, and each signal can thus have a desired characteristic (e.g., energy characteristic, angle of incidence, etc.).
In general, providing an x-ray tube that is capable of generating multiple focal spots of specific size and shape has proven difficult. One approach is to utilize an x-ray tube having multiple cathode head structures. With this approach, a separate cathode with its own cathode cup, heated filament and electrical circuit, is provided. Each cathode is then physically oriented with respect to the anode target surface in a manner so as be capable of generating a separate focal spot. While this approach does result in the generation of multiple x-ray signals, it is not entirely satisfactory for several reasons. It requires additional structural components within the x-ray tube, which increases manufacturing cost and complexity, and increases the likelihood of component failure. Moreover, the number of focal spots that can be produced is limited by the number of cathode structures provided, thereby limiting the number and types of x-ray signals that can be produced.
Another approach for producing multiple x-ray signals is to provide some facility for redirecting or displacing the point of impact of the electron beam (i.e., the focal spot) to different positions on the focal track. These approaches typically utilize a voltage potential to deflect the electron beam after it has been emitted from the cathode filament. However, x-ray tubes using these approaches have not been entirely satisfactory either. For instance, in designs of this sort a deflection mechanism, such as multiple deflection plates, is usually disposed external to the cathode. In operation, a voltage potential is applied to the deflection plates, which creates a deflection region between the cathode and the anode target. Typically, one plate is placed at a much higher negative voltage with respect to the other deflection plate. This voltage bias acts to deflect and alter the direction of the accelerating electron beam, and thus causes it to impinge on a different focal spot location on the anode target surface.
The use of these deflection plates cause several problems that can negatively affect the quality of the resulting x-ray signal. First, in some designs the deflection plates are positioned external to the focusing structure of the cathode cup. Thus, the electron beam has already been formed and focused, and is accelerating towards the anode before it reaches the deflection region. At this point, the electrons are already traveling at a high rate of speed and have therefore achieved an appreciable amount of energy. As such, deflection of the electron beam to alter its direction requires that a high voltage potential be applied to the deflection plates. However, higher voltage can result in arcing between the deflection mechanism and the anode structure, which can render the tube inoperable. To alleviate this problem, the anode must be physically spaced farther from the cathode structure. However, moving the target farther from the anode results in lower x-ray emission, thereby decreasing the quality of the x-ray image. This is not acceptable in many applications. Designs utilizing external deflection plates must thus limit the amount of voltage potential used to steer the electron beam (to maintain the stability of the tube and avoid electrical arcing). This limits the degree to which the electron beam can be deflected. Alternatively, such designs must increase the distance between deflection plates and the anode, which decreases the x-ray emission quality due to the resulting increase in distance between the anode and the cathode.
Another problem with the use of such external deflection plates is that the physical position of the plates relative to one another and relative to the cathode cup and filament, can greatly affect the ability to precisely steer the electron beam. However, each of the plates is typically supported by a separate support structure. Thus, mechanical precision is difficult to achieve, and can result in an expensive and time consuming, manufacturing and assembly process. Moreover, repeated use of the x-ray tubexe2x80x94especially in the extreme thermal and vibrational conditions of an operating x-ray tubexe2x80x94can cause deformation of the deflection plates relative to one another. This reduces the operational efficiency of the tube, and can result in a tube having a shorter operational life.
As can be seen, the problems encountered when using external plates are due in large part to the physical distance between the plates and the electron emission source, or filament. However, moving the plates closer to the filament creates other problems, namely, by adversely affecting the emission region of the filament. This is due primarily to the manner in which electrons are emitted, or xe2x80x9cboiledxe2x80x9d off, from the filament. In generals electrons are boiled off from the filament at a minimum energy level, which is dependent on the filament material (e.g., approximately 4.5 eV for tungsten). If after being boiled off the filament the electrons encounter a retarding field with greater than this minimum exit energy, the electrons are returned to the filament, forming an electron cloud. This circumstance affects the transmission qualities of the electron beam that is accelerated towards the anode target, e.g., the emission region can narrow and/or shift. In contrast, if the electrons immediately encounter in accelerating field, they accelerate towards the target and gain energy. The resultant beam has minimal electron emission variation from the filament. Consequently, positioning the deflection plates close to the filament can result in a diffuse electron source that compromises the focusing capability of the cathode structure.
Examples of this, as well as other problems can be seen in those x-ray tube designs that have attempted to address the problems inherent with the use of external deflection plates by moving the deflection function closer to the electron source. For example, one approach is to eliminate the use of separate physical deflection plates, and instead deflect the electron beam with the cathode focusing cup itself. These designs essentially split the cathode focusing cup into different and electrically insulated parts, and then apply the voltage bias to the separate parts so as to deflect the electron beam. Thus, there is no separation of the focusing and deflection functions, insofar as both are provided within the cathode focusing cup itself. This focusing and deflection of the electron beam with the same structure reduces the ability to provide a well controlled electron beam and tightly controlled and focused focal spot at the anode target. For example, there is no ability to independently focus, control, modify and/or deflect the electron beam trajectory or shape since all of this is done simultaneously within the same cathode structure. Thus, there is no ability to allow separate control over the electron beam parameters and focal spot size and/or dimensions. Also, in operation, only the filament portion of the cathode structure is at xe2x80x9ccathodexe2x80x9d potential, and the remaining parts of the cathode are at a specified deflector bias potentials. Thus, such a structure has varying bias voltages and varying electron emission levels depending on the applied AC voltage. Moreover, because of the proximity to the electron filament source, the electron emission levels will also vary depending on the applied deflector bias. Also, the electron optics provided by such a structure are complicated and difficult to control and define due to the moving electron source region of the filament, which is again affected by the particular deflector bias. For instance, as noted above, when a bias is applied in such devices, the emission region of the electron beam typically narrows and shifts.
To address the problems resulting from integrating the deflector function into the cathode cup itself, some x-ray tubes utilize deflectors that are attached directly to the cathode cup focusing device via an insulator. However, such an approach still has the stability problems found in devices using separate deflector plates (i.e., less tube stability due to arcing between the grids and the cathode); and also have some of the same problems encountered in approaches integrating the function within the cathode cup, i.e., reduced emissions and space charge limitations. Since the deflector grids are only separated from the focusing cup by an insulator, the plates still compromise the focusing ability of the cathode structure. Again, the electrons emitted from the filament immediately encounter a retarding field created by the bias applied to the deflector plates that is negative with respect to the cathode. This creates emission and space charge limitations that limit the focusing ability of the cathode. Moreover, the length of the deflector plates along the beam axis cause a lensing action, which is due to the curvature of the electric field lines which penetrate into the filament opening. This further reduces the focusing capability of the cathode structure.
Because of the problems with tube stability, and the reduced focusing capability of the cathode structure, previous cathode designs for providing adjustable focal spots have not been entirely satisfactory. As noted, in many applications the specific distribution of the electron beam on the target focal spot, as well as the intensity distribution of the focal spot, are extremely important. A precisely focused electron beam is important for providing an x-ray signal with optimum beam quality, which in turn enhances the quality of the resulting x-ray image. As highlighted above, in many of the existing designs the deflection of the electron beam can degrade the quality of the xe2x80x9cfocusxe2x80x9d of the impinging electron beam, including the shape and intensity of its distribution on the target surface. This can decrease the quality of the x-ray signal and resulting image.
Existing designsxe2x80x94regardless of the deflection mechanism and scheme usedxe2x80x94suffer from yet another substantial problem. In particular, the deflector structures of the prior art are subjected to extreme thermal conditions that can damage the cathode structure and limit the operational life of the x-ray tube. As noted when the electron beam impinges on the target location of the anode, a small percentage of the resulting kinetic energy is released as x-rays. However, a substantial portion of the kinetic energy is converted to extremely high levels of heatxe2x80x94upwards of 2500xc2x0 C.xe2x80x94which is radiated from the anode target. Some of this heat is absorbed into other parts of the x-ray tube, including the proximally located deflector portions of the cathode structure in the above-described prior art tubes. This imposes a large amount of thermal stress on the structure that can damage the cathode and limit its overall operating life.
Thus, there is a need in the art for an x-ray tube that is capable of generating multiple focal spots at different positions on the anode target, and thereby produce multiple x-ray signals having varying angles of incidence and/or energy distributions. In addition, the x-ray tube should be capable of providing precise control over the size, shape and energy distribution of each of the varying electron focal spots. It would also be advantageous to provide an x-ray tube that minimizes any electron emission variation from the filament under changing deflector bias and anode-cathode voltage and configuration conditions. It would also be an advancement in the art to provide an x-ray tube that is stable, and that is not prone to arcing between the anode and cathode, even in the presence of large voltage biases for displacement of the electron beam. Preferably, the x-ray tube would include a cathode assembly that is better able to withstand the extreme thermal stresses imposed by heat radiated from the anode target.
It is therefore a general objective of the present invention to provide an improved x-ray tube that is capable of producing multiple x-ray signals having varying angles of incidence and/or energy distributions by varying the focal spot position on an x-ray tube target.
More particularly, it is one primary object of the present invention to provide an improved cathode structure for use in an x-ray tube, that is capable of varying the direction of the electron beam so that it impinges at different focal spots on the anode target.
Another objective of the present invention is to provide an improved cathode structure that is capable of maintaining precise control over the shape, size and energy distribution of the focal spot formed by the electron beam on the target anode.
Yet another object of the present invention to provide an improved x-ray tube that is stable over a wide operating range. More particularly it is an objective of embodiments of the invention to provide a cathode structure that is stable, even at high voltage potentials between the cathode structure and the anode. Similarly, it is an objective of certain embodiments of the present invention to provide a cathode structure that is capable of redirecting an electron beam with deflectors that can be placed at high bias voltages without causing electrical arcing to occur between the cathode and the anode.
Another object of the present invention is to provide an x-ray tube that allows for the production of varying focal spots and that yet minimizes any electron emission variation from the cathode filament, and which thereby maintain the focusing capability of the cathode. More particularly, it is an objective of embodiments of the present invention to provide an improved cathode structure that reduces electron emission variation even under changing deflector bias voltages.
Still another object of the present invention is to provide an x-ray tube that is more resistant to high temperatures produced during operation of the tube. More particularly, it is an objective of embodiments of the invention to provide a cathode structure that is protected from the extreme temperatures radiated from the anode target during operation.
Other objects and advantages of the invention will become apparent upon reading the following detailed description and appended claims, and upon reference to the accompanying drawings.
Briefly summarized, these and other objects, features and advantages are provided with an improved x-ray tube. Generally, the x-ray tube includes an anode structure and a cathode structure that are each disposed within an evacuated tube. The anode includes a focal track, or similar anode target area, that, when impinged with electrons emitted from the cathode, generates x-rays.
In a preferred embodiment, the x-ray tube includes an improved cathode structure, which is capable of providing at least two important functions. First, it provides for the emission of an electron beam that creates a focal spot on the anode target that has precise dimensions, shape, size and electron distribution. Precise control over these local spot characteristics results in the production of an x-ray signal that provides an improved x-ray image. Secondly, embodiments of the improved cathode structure allows for the production of multiple focal spots on the anode target at varying positions. In this way, x-ray signals having different intensity levels, and/or varying angles of incidence can be produced, depending on the position of the focal spot on the anode target.
In a preferred embodiment, the cathode structure includes a means for emitting electrons, such as a single filament that, when heated, discharges electrons. The preferred cathode structure further includes a primary means for focusing the electrons emitted from the filament, such as a cathode focusing cup. This cathode cup is supported on a cathode support base structure, which provides support to the entire cathode assembly within the evacuated tube relative to the anode target. In one preferred embodiment, the cathode cup is comprised of two focusing arms disposed on opposite sides of the filament. Preferably, each of the focusing arms of the cathode cup are electrically connected so as to be placed at a cathode voltage potential, which is substantially equal to the voltage potential of the filament. During operation, the anode is placed at the anode voltage potential, and electrons emitted from the heated filament are accelerated towards the anode target. The focusing arms of the cathode cup have outer surfaces that are oriented in a manner so as to focus and shape the electron fields at the filament, and deflect electron trajectories in the back side of the filament.
In preferred embodiments, the cathode structure further includes a secondary means for focusing the electron beam that is emitted from the cathode structure. For example, in one embodiment, the focusing means is comprised of a focusing aperture formed in a cap structure. The cap structure can be formed as a hollow cylinder that substantially encloses the cathode cup and filament. Formed within a top surface of the cap is the focusing, aperture. The focusing aperture is positioned relative to the cathode cup and the filament so that the accelerating electrons pass through the aperture. The focusing aperture is of a size and shape that further. Focuses the electron beam so as to obtain a focal spot that has predefined characteristics. Preferably, the cap structure is at the same voltage potential as the cathode cup, and is structurally supported by the cathode support arm.
In preferred embodiments, the cathode structure also includes means for creating a deflection region between the cathode cup and the focusing aperture. This deflection region alters the trajectory of the electron beam, thereby causing the position of the focal spot on the anode target to shift accordingly. In one preferred embodiment, the deflection means is comprised of two deflector grids or plates that are disposed on opposite sides of the filament, and at a point above the cathode cup focusing arms. The plates are also disposed within the interior housing formed by the cap structure. Each deflector plate is supported by a separate dielectric support means, each of which are connected to and supported by the cathode support base. Each dielectric support means electrically insulates each deflector plate from the rest of the cathode structure, including the cathode cup. Each deflector plate is electrically connected to a voltage source, which is used to apply a bias potential of sufficient magnitude to each plate that deflects the trajectory of the electron beam. This deflection of the beam direction causes a corresponding shift in the focal spot position on the focal track.
The present cathode structure provides a variety of advantages over the prior art. In particular, the dual focusing arrangement provided first by the cathode cup focusing elements, and second by the focusing aperture, provide an increased level of focusing and control over the electron beam and resulting focal spot. Moreover, the cathode cup provides an electron beam that has very little emission variation from the filamentxe2x80x94even in the presence of an applied potential at the deflector plates. Consequently, a focal spot having precise dimensions, shape and electron distribution is obtained, resulting in an improved x-ray image. In addition, embodiments of the cathode structure provide precise control of the focal spot position on the anode target. This is accomplished, for instance, with deflector plates that are separate and distinct from the focusing elements of the cathode. Moreover, increased deflection bias potentials can be utilized to more precisely control the trajectory of the beam without causing electrical arcing between the cathode and the anode. This is due to the cathode cap structure, which is at a fixed potential and is positioned between the cathode structure and the anode. This reduces arcing between those two elements and increases the overall stability of the x-ray tube. Moreover, heating of the deflector plates, the cathode filament and the cathode cup from heat radiated from the anode surface is greatly reduced by the presence of the cathode cap. This reduces the thermal stresses present, and increases the reliability and operating life of the cathode structure.